An SSB radio communication system is widely used, wherein wireless communications are carried out by using amplitude modulation signals.
In this SSB radio communication system, a carrier wave and one of upper and lower side band waves that do not contribute directly to the transmission of information are suppressed, and only either one of the upper and lower side band waves is transmitted. This system not only saves electric power but also can use a number of communication channels by using only a half the frequency band.
FIG. 1 shows a block diagram of a receiving circuit for a radio apparatus using the conventional SSB radio communication system.
In FIG. 1, an antenna 11 receives single side band signals transmitted from the transmitting side, and a high frequency amplifier 12 amplifies the received signals. A frequency converter 13 mixes the single side band signals from the high frequency amplifier 12 and a signal from a local oscillator 14, and converts the mixed signals through a band limiting filter 15 into intermediate frequency signals. In an intermediate frequency amplifier 16, the gain of the converted signals is automatically adjusted by a detector 17 so as to give an output of a predetermined value. In a demodulator 18, the automatically gain adjusted intermediate signals are demodulated based on a signal from a local carrier wave oscillator 19. The output signals of the demodulator 18 are passed through a low pass filter 20 and amplified by a low frequency amplifier 21 to obtain demodulated output signals.
However, since in this SSB radio communication system there is no reference carrier wave signal serving as a reference for adjusting a level when an automatic gain adjustment is carried out in the intermediate frequency amplifier 16 and detector 17 of FIG. 1, the time constant in the automatic gain adjustment needs to be made sufficiently longer than the period of the modulated signals. For this reason, when the level of the received signals changes, depending on the conditions of a radio communication path, at a period shorter than that in the automatic gain adjustment, an amplitude distortion will be generated in the receiving circuit so that the received output changes.
In addition, to completely recover the waveforms of the modulated signals, it is needed that a frequency which causes frequencies in the transmitting and receiving sides to coincide must be fed to the frequency converter 13 and demodulator 18 from the local oscillator 14 and local carrier wave oscillator 19, respectively. Even if a little error is allowable, a frequency within the allowable range must be supplied. Thus, to coincide frequencies in the transmitting and receiving sides a high stable oscillator is required. When there is no such a high stable oscillator, for example, a circuit (clarifier) for finely adjusting the frequency in the receiving side will be needed.
Further, since in the SSB radio communication system the output level of the received signal changes depending on the conditions of a radio communication path, it is difficult to handle the values of amplitudes as the values of modulated digital signals so that the efficient transmission of data is impossible.